'aTJcK-JSS'i'iiRE B&BIOLOGXC:*li B^L^: kftSOa
aY TECHNICAL REPORT '3IUMBBR 3
November 15, 1963
COIgggRCCT I_Q OF_TS?10ggR.IC . TJETgMQR
Erich Bretthauer
Frank Williams
Bernard Furaagalli
SUMMARY
"Thermornetric titrimatry, " a versatile and effective
technique for the analytical chemist,, utilizes heats of
reaction to obtain titratiors. curves. In conventional types
of titrations (e.g., potentiornetric, conductometric-
coiorimetriCj ate.}, determination of end points depends on
free energy effects. The rxnome trie titration curves, however,
represent a measure of the entire enthalpy change which
includes the entropy as well as the free energy involved.
The inethcd permits analysts to carry out, accurately
and rapidly „ many quantitative determinations not previously
possible .
It was be.lieved that a thercmonsetric titrator would
permit this laboratory to study certain reactions, i.e^
SBT& determination of calcium „ which could not be studied by
conventional means „
This paper describes the construction of a therracmetric
titrator and certain improvements made upon the only existant ,
commercial model-
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Cn. March 31, ¥& , Pysel a&d f-ir. 3rettlhauer attendee the
Chemical Xnstruraent Show in Los Angeles. Cn demonstration
was a tliermochemical titrator, made by the American 'J-nstrisiite^t
Compassy .
"Thermoffieiric titrimetry, " a versatile and effective
technique for the analytical chemist, utilizes heats of
reaction to obtain titration curves. In conventional types
of titrations {e.g., potent iometric, conductowstric,,
color inetric,, stc=)j determination of end points depends
on free energy effects. They depend solely on equilibrium
constants; which are correlated with free energies by the
equation
-&F° = ET In K
where AP° denotes the standard free energy and K the equilibrium
constant o
The reaction enthalpy is one of the most general properties
of a chemical reaction
AH == AF + TAS
Saspaction of this equation reveals that analytical
prroeeeiares based on heats of reactions may be 'workable when
all free energies Methods fail. Thermometric titration
curves, there fore ,, represent a measure of the entire enthalpy
change which includes the entropy as well as the free energy
The method permits analysts to carry out, accurately and
rapidly 0 many quantitative determinations not previously possible.
rrhe therr-ioiTsetric titrator rnade by the American Znstricment
GompsKy makes use of a thermistor for measuring temperature.
c
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theruiistor is than incorporated into one arr,: of a friheacstciia
bridge, thereby giving direst ealorisr.etrie data,
"flex* tltsraietitrator marketed by the American Znstrtame&t
my has bseii available since January of 19S30 This
al model retails for about $1600 and consists of a
thermistor iftheatstone bridge circuit, adiabatic tower with
polynsrethasie insulated titration cell, teflon stirring bar
attached directly to a synchronous motor, glass encapsulated
1
hsater, and a constant flow semi-raicico bureto. It was
felt that a thermoehemical titsrator would offer this
laboratory the ability to evaluate pertinent reaction
parameters heretofore unavailable and that such a titrator
eo^ld be constructed x^ith a sensitivity at least as
great as the ccmrcercial instruBaesjt for a fraction of the cost,
On Jxans 12, 1963, construction x-7as begun on a model to
ba q^ite similar to that marketed cosamercially, As will be
showiii bslow, hctfeve^j due to certain necessary modifications
the fisnislTsd raodel t-^as remarkably different than envisaged»
A schematic of the VJheatstone bridge circuit is shewn in
F;lg0 10 It was found that the circuit did not have the
requisite stability in air0 A much greater degree of
stability was attained when the circuit was suspended in an
oil 'sathp thereby minimising any ehaziges in resistance «5Stae to
rapid changes in room temperature.
Figure 1
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The titration cell was initially constructed of glass
completely insulated in one inch of polyurethane. This cell
was therefore quite similar to that found in the thermal
titrator made by the American Instrument Company a However,,
due to the rapid temperature changing environment found in
this laboratory, the cell did not provide adequate adiabatic
character. The rapid changing temperature environment also
provided great difficulty in keeping the titrant and titration
solution initially at the same temperature- These difficulties
were largely overcome by designing a titration cell with an
attached titrant reservoir (Fig. 2), which was suspended in a
large volume of oil. Figure 3 displays the thermistor-stirrer-
heater assembly included in the titration cell.
Celt
eservoir
\
Hecdrer Wire,
Figure 2 Figure 3
The stirrer was initially constructed with a teflon
stirring bar attached to a synchronous motor. This also was
identical to the stirring mechanism of American Instrument
Company°s Thermal Titrator. It was found, however, that a
considerable amount of heat was conducted from the motor
down the teflon bar into the reaction solution. This
extraneous heat was eliminated by a belt driven stirrer.
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It was also found that the sharpness of a
during calibration tests was improved slightly by not
encapsulating the heater wire. Of course, if corrosive
solutions are used; encapsulation is necessary„
^he thermal titrator, complete with an automatic semi-
2
micro fouret purchased from American Instrument,, is shown
in Fig* 4.
Figure 4
The sensitivity of the instrument is about the same as
the commercial model„ However, due to elimination of certain
disadvantages discussed above, it is felt that this instrument
has greater accuracy and versatility„ Ho degree of improvement
is certified for this instrument as comparison is difficult
without availability of the commercial instrument,
BIBLIOGRAPHY
1= American Instrument Co.;, Inc., (8030 Georgia Ave», Silver
Spring, Md.)» "Titra-thermo-mat, Automatic Therraometric
Titrator," Bulletin 2375, 1962.
2. American Instrument Co** Inc., (8030 Georgia Ave=5 Silver
Spring, Md.J, "Menisco-matic Buret0" Instruction Wo= 774 Ec 1963,
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ACKNOWLEDGEMENT
The author is indebted to Messrs, Ewald Pyzel, Earl
Whittaker0 Robert Griffin, James Dillon and Ray Rawson for
their technical assistance0
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